1. Field of the Invention
The present invention concerns a device and a method for electrical charging of a transport belt for transporting recording media in a transfer printing region of an electrophotographic printer or copier device. The invention also concerns an associated blade-like contact element.
2. Description of the Related Art
In electrophotographic printers or copier devices, the transfer of a toner image from an the intermediate carrier (for example a photoconductor drum or a photoconductor belt) onto a the recording medium is referred to as transfer printing. The section of the printer or copier device at which the intermediate carrier and the recording medium are brought into contact with one another is referred to as a transfer printing region. In the transfer printing region, the intermediate carrier (for example the generated surface of a photoconductor drum) and the recording medium move with the same speed in the same direction while the toner is transferred from the intermediate carrier onto the recording medium.
A good print image on the recording medium can only be achieved when a uniform contact is produced between the recording medium and the intermediate carrier in the transfer printing region. A good and uniform contact between the recording medium and the intermediate carrier can be achieved with the aid of an electrostatically-chargeable transport belt on which the recording media lies and, adhering to this with electrostatic forces, is transported through the transfer printing region.
A device for transfer of a toner image with the aid of an electrostatically-chargeable transport belt is shown in German Patent Document DE 102 47 368.4 (not previously published), which is incorporated by reference into the present specification. In this device, the transport belt is charged with a charge whose polarity is different than the polarity of the charge of the toner image. This electrostatic charging of the transport belt has a two-fold function: on the one hand, it effects an electrostatic attraction of the recording medium to the transport belt and thus a secure guidance of the recording medium in the transfer printing region; on the other hand, it effects the transfer of the toner image from the intermediate carrier onto the recording medium.
Similar devices with electrostatically-charge transport belts are also known from U.S. Pat. No. 5,666,622, German Patent Document DE 195 01 544 A1 and U.S. Pat. No. 5,159,392. In these three documents, the transport belt is either charged via corona arrangements (what are known as corotrons) or via contact rollers. A corotron typically comprises one or more thin, gold-coated tungsten wires whose electrical potential is about 1000 V relative to a grounded housing, such that the air surrounding the wires is ionized.
However, corotrons have a number of serious disadvantages including, for example, the ozone formation due to the high charge voltage and the relatively complicated exchange of worn-our corotron wires. Moreover, the corotron wires are easily contaminated with dust, belt abrasion particles and toner particles, which leads to an irregular charge distribution on the transport belt. Locations with a lower transport belt charge lead to a less complete transfer of the toner onto the overlying the recording medium and thus to unwanted lightening, or fading, of the print image. The cleaning of the corotron wires is not only elaborate but also subjects the wires to a significant mechanical stress and shortens their lifespan.
Contact rollers also have a disadvantage in that they can become contaminated easily and thereby lead to an irregular charging of the transport belt. Moreover, they can not be directly arranged in the transfer printing region because they would interfere with the uniform arrangement of the recording medium on the intermediate carrier. Nevertheless, in order to achieve a sufficient charge of the transport belt in the transfer printing region, a certain current must flow at the contact point from the contact roller and transport belt to the transfer printing region. Therefore, the conductivity of the transport belt may not be too low, which represents a disadvantageous limitation for the selection of the transport belt material used.